This invention relates to processes and apparatus for locating sources of charged particles, and more specifically to such processes and apparatus having high spatial resolution and efficiency.
The need to identify the components of unknown mixtures is faced in a wide variety of situations. One way to meet this need is physically to separate the components, and many separation techniques are in common use in science and industry. Most separation techniques exploit differences in the physical/chemical properties of the components of the mixture. The components of a mixture that have been separated into agglomerations of like molecules or particles can then be recognized by their locations in space. Frequently, the component particles are labeled with a radioactive substance to facilitate the determination of their spatial locations.
The location of each radio-labeled component (agglomeration of like particles) of a mixture can be determined using a number of techniques. If the components are separately disposed on, or through, a solid or semisolid medium, the medium might be divided into pieces and the radioactivity of each piece directly determined. This method provides adequate quantitative results, but its resolution is limited in practice by the minimum size of slices that can be used. Also, it may not be practical to count a very large number of slices.
A number of position-sensitive radiation detectors are known. Some use multiple closely spaced detectors to determine the location of a source of incident radiation; others use position-sensitive counters developed for use in high energy physics experimentation. This latter type of apparatus typically uses a wire or a grid of wires disposed in an ionizable medium (liquid or gas). When a charged particle emitted by the radioactive label passes through the medium, it triggers an avalanche of charges (typically electrons), which induces a pulse on an electrical delay line. The timing of the pulse is used to infer the location of the source of the incident charged particle. Such detectors are described in U.S. Pat. Nos. 3,483,377 and 3,517,194 to Borkowski et al.; 3,911,279 to Gilland et al.; 3,975,638 to Grunberg et al.; and 4,149,109 to Kreutz et al.; and in Hendrix, "One and Two Dimensional Position-Sensitive X-Ray and Neutron Detectors", Trans. Amer. Crystallographic Ass'n vol. 12, pp. 103-146 (1976).
To improve the spatial resolution of such devices (viz., the ability to recognize two close sources as distinct), electronic controls that limit pulse length or collimators that limit the angular extent of particles admitted to the detector have been added. Such detectors are described in U.S. Pat. Nos. 3,898,465 to Zaklad et al.; 3,975,639 to Allemand; 4,311,908 to Goulianos et al.; and 4,707,608 to DiBianca.
Another well known type of position-sensitive radiation detector employs photographic film, and is described in Laskey et al., "Quantitative Film Detection of .sup.3 H and .sup.14 C in Polyacrylamide Gels by Fluorography" Eur. Biochem. vol. 56, pp. 335-341 (1975), for example.
The fact that radioactive sources spontaneously emit radiation into 4.pi. steradians leads to two problems in the prior detectors, the first of which is limited spatial resolution. Prior position-sensitive detectors typically collect the spontaneously emitted radiation through a "window" in a counting tube or like component. Thus, the ability of the detector to determine where the source is located is inherently limited by parallax. Moreover, the farther the detector must be from the source, the larger must be the distance between the sources, e.g., two components of a mixture, for the detector to recognize them as separate.
These effects are illustrated in FIG. 1, which depicts three point sources 1-3 and a detector 4. The sources spherically emit radiation, indicated by the arrows, portions of which impinge on the detector. Although the sources 1-3 are well separated, the detector 4 has difficulty resolving sources 1 and 2 and sources 2 and 3 because radiations from the sources overlap on the detector.
Placing the detector as close as possible to the source and limiting the size of a window between the detector and sources can increase the spatial resolution to as good as .+-.0.5 mm, depending on the nature of the radioactive source. In gas-ionization detectors that amplify the radio-emitted particles by creating an electron avalanche, such as the detector described in U.S. Pat. No. 5,083,027 to Kuhn, devices that shape or collimate the ionization avalanche can also be included.
Nevertheless, using a small window worsens the prior detectors' second problem, which may be called limited efficiency. The fact that most of the radio-emitted particles do not impinge on the detector (see FIG. 1) is an inherent limitation on the detector's efficiency. Additional losses result from the incorporation of electronic controls on pulse length.
Moving a windowed detector closer to a source improves spatial resolution by reducing parallax between the source and the window. Charged particles entering the counting "chamber" are detected through the electron avalanches they trigger, but avalanche electrons moving at many angles away from the detector are either lost or broaden the source image. Moving the detector closer to the window within the counting chamber may reduce parallax but it also decreases efficiency. The closer the window is to the detector, the less likely it is that the small number of radio-emitted charged particles moving in the direction of the detector will collide with ionizable gas molecules in the chamber and induce avalanches.
The above-cited Kuhn patent describes an apparatus that improves the degree of locational resolution by placing the source in a test piece within the counting chamber (viz., the window is eliminated). Another fundamental aspect of the Kuhn apparatus is its use of mechanical collimators that improve resolution by shaping the electron avalanche. The Kuhn apparatus relies on a location-sensitive proportional counting tube that uses an electron avalanche induced in an ionizable gas by passage of a radio-emitted charged particle. The spatial resolution of such a system increases as the aperture of the collimator decreases, but the detector's efficiency decreases at the same time.
The system described in the Kuhn patent has a drift field, as in a number of other position-sensitive counters, that directs the avalanche electrons to the detector. Avalanche electrons moving at many angles away from the detector are either lost or drift irregularly towards the detector, broadening the source image.
In view of these problems with the previous detectors, there is a clear need for a detector that has both high spatial resolution and efficiency.